Size of reference to std::array not available at compiletime

Question

I'm interested to know why the second static_assert in my code below doesn't work. It seems like even though the array c is a reference to a, the size of the array is embedded in the type, so it should be available at compile time.

#include <array>

int main()
{
    std::array<int,2> a = {1,2};
    std::array<int,2> b = {2,3};
    std::array<int,2>& c = a;

    static_assert(a.size() == b.size(), "a.size==b.size"); // ok size of array is compile time constant
    static_assert(c.size() == a.size(), "c.size==a.size"); // compiler error "static_assert expression is not an integral constant expression"
}

Answer 1

the size of the array is embedded in the type, so it should be available at compile time.

This is true. But regardless, c is not a constant expression and therefore expression that contains it as a subexpression cannot be a constant expression - except certain operators that interact only with the type of the operand such as sizeof.

You can get the size for example using:

static_assert(
    std::tuple_size<
        std::remove_reference_t<decltype(c)>
    >::value == a.size(),
    "c.size==a.size"
);

Unfortunately, it is not very pretty.